Competitive Exams Floods: Forecast, Prediction & Assessment Predicting, forecasting and assessing floods are important topics for

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Floods & Droughts in India

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Flooding Hazards, Prediction & Human Intervention Hazards Associated With Flooding

Hazards associated with flooding can be divided into primary hazards that occur due to contact with water, secondary effects that occur because of the flooding, such as disruption of services, health impacts such as famine and disease, and tertiary effects such as changes in the position of river channels. Throughout the last century, flooding has been one of the most. Costly disasters in terms of both property damage and human casualties.

Primary Effects

  • Again, the primary effect of floods are those due to direct contact with the floodwaters. Water velocities tend to be high in floods. As discharge increases velocity increases.

  • With higher velocities, streams are able to transport larger particles as suspended load. Such large particles include not only rocks and sediment, but, during a flood, could include such large objlOcts as automobiles, houses and bridges.

  • Massive amounts of erosion can be accomplished by flood waters. Such erosion can undermine bridge structures, levees and buildings causing their collapse.

  • Water entering human built structures cause water damage. Even with minor flooding of homes, furniture is ruined, floors and walls are damaged, and anything that comes in contact with the water is likely to be damaged or lost. Flooding of automobiles usually resul ts in damage that cannot easily be repaired.

  • The high velocity of flood waters allows the water to carry more sediment as suspended load. When the flood waters retreat, velocity is generally much lower and sediment is deposited. After retreat of the floodwaters, everything is usually covered with a thick layer of stream deposited mud, including the interior of buildings.

  • Flooding of farmland usually results in crop damage. Livestock, pets, and other animals are often carried away and drown.

  • Humans that get caught in the high velocity flood waters a re often drowned by the water.

  • Floodwaters can concentrate garbage, debris, and toxic pollutants that can cause the secondary effects of health hazards. Secondary and Tertiary Effects: Secondary effects are those that occur because of the primary effects and tertiary effects are the long term changes that take place. Among the secondary effects or a flood are:

  • Disruption of service so Drinking water supplies may become polluted, especially if sewerage treatment plants are flooded. This delay result in disease and other health effects, especially in under developed countries. o Gas and electrical service may be disrupted.

  • Transportation systems may be disrupted, resulting in shortages of food and clean-up supplies. In under developed countries food shortages often lead to starvation.

Long - term effects (tertiary effects)

  • Location of river channels may change as the result of flooding, new channels develop, leaving the old channels dry. a Sediment deposited by flooding may destroy farm land (although silt deposited by floodwaters could also help to increase agricultural productivity).

  • Jobs may be lost due to the disruption of services, destruction of business, etc. (although jobs may be gained in the construction industry to help rebuild or repair flood damage).

  • Insurance rates may increase. Corruption may result from misuse of relief funds. Destruction of wildlife habitat.

Predicting River Flooding

Floods can be such devastating disasters that anyone can be affected at almost anytime. As we have seen, when water falls on the surface of Earth, it has to go somewhere. In order to reduce the risk due to flood s, three main approaches are taken to flood prediction. Statistical studies can be undertaken to attempt to determine the probability and frequency of high discharges of streams that cause flooding. Floods can be modeled and maps can be made to determine the extent of possible flooding when it occurs in the future. And, since the main causes of flooding are abnormal, amounts of rainfall and sudden thawing of snow or ice, storms and snow levels can be monitored to provide short-term flood prediction.

Monitoring the Progress of Storms: If factors such as amount of rainfall, degree of ground saturation, degree of permeable soil, and amount of vegetation can be determined, then these can be correlated to give short-term prediction. in this case called a forecast. of possible flood s. If a forecast is issued, then a flood warning can be communicated to warn the public about the possible extent of the flood, and to give people time to move out of the area. Such forecasts are very useful for flooding that has a long lag time between the storm and the peak discharge. Flash floods, which characteristically have s hort. lag times, are more problematical. Thus, in some areas known to be susceptible to flash floods, a flash flood warning is often issued any time heavy rainfall is expected because there is always the chance of a accompanying heavy rainfall.

Human Intervention

Humans can modify landscape in many ways. Sometimes humans attempt to modify drainage systems to prevent flooding, but sometimes these efforts have adverse effects and actually help to cause flooding in ether areas. Any modification of the landscape has the potential to cause changes in the drainage system. and such changes can have severe consequences.